It is known that titanium dioxide particles present as the pigment component in a surface-coating composition cause oxidative decomposition of the polymer on exposure to ultra-violet rays and moisture, known as whitening. In order to suppress this effect of titanium dioxide, it has been proposed to coat or dope titanium dioxide with compounds of chromium, silicon, aluminum, zinc, phosphorus or zirconium.
EP-A-0 268 918 describes a weathering-resistant pearlescent pigment having a hydrated zirconium oxide coating on the titanium dioxide base pigment, this coating being obtained by hydrolysis of a zirconium salt in the presence of a hypophosphite.
EP-A-0 342 533 describes a weathering-resistant pearlescent pigment having, on the titanium dioxide base pigment, a top layer comprising hydrated zirconium oxide obtained by hydrolysis in the presence of a hypophosphite, and a hydrated metal oxide. The metal oxide can be cobalt oxide, manganese oxide or cerium oxide.
These modified pearlescent pigments have adequate dispersibility and weathering resistance in non-aqueous surface-coating systems. However, they are not suitable for use in water-thinnable surface-coating systems, since they cause the formation of microfine bubbles in the coating film which significantly increases light scattering and thus adversely affects gloss and color. In addition, the distinctness of image (DOI) is greatly reduced and the regeneration capacity of the coating film is impaired.
The Huls AG company brochure "Dynasylan.RTM.-Anwendungen yon organofunktionellen Silanen" [Dynasylan.RTM.-Applications of Organofunctional Silanes] discloses silanes as coupling agents between inorganic materials (glass, minerals and metal) and organic polymers (thermosets, thermoplastics and elastomers). In a composite material comprising an inorganic substrate and a polymer, they improve the adhesion and thus the mechanical properties of the composite and, simultaneously, as surface modifiers, hydrophobicize the surface of the substrate and thus improve the wetting by the polymer.
The organofunctional silanes have the following basic structure EQU Y--(CH.sub.2).sub.n --SiX.sub.3,
where Y is an organofunctional group, for example an NH.sub.2 group, which is rigidly bonded to the silicon via a carbon chain. X is referred to as a silicon-functional group. These are generally alkoxy groups, which, after hydrolysis, react with active sites of an inorganic substrate or with other silicon compounds by condensation and thus form stable bonds.
Attempts to modify the surface of pearlescent pigments with the aid of these organofunctional silanes in order to improve their weathering resistance and to make them suitable for water-thinnable surface-coating systems have given unsatisfactory results. The strength of the bond between the silane and the surface of the substrate is limited. Mechanical stress or weathering effects, as occur, for example, in an automotive paint, break the bond. A further disadvantage is that the coverage of the substrate surface by the silane is incomplete.